1. Field of the Invention
This invention relates, in general, to printing devices and, more specifically, to laser printers utilizing multiple-spot exposure of a photosensitive member.
2. Description of the Prior Art
Many traditional laser printers use a single laser beam which is modulated and scanned across the surface of a photosensitive member. The selective exposure of a photosensitive member provided by this system creates a latent image which can be developed and transferred to the hard copy output medium, such as a sheet of paper. Since the beam must scan the entire page area to create the image, the throughput or speed of the printer is dependent upon the scanning time.
In order to produce faster printers with comparable quality outputs, some printers use more than one modulated laser beam in the scanning and exposure process. For example, a system which uses three simultaneously modulated laser beams which are scanned together across the photosensitive surface or member can expose the member in one third the time required by the single laser scanning system. Each of the three beams can scan adjacent lines in the image being created or they can be interlaced with other lines to be scanned in subsequent scan passes. The individual laser beams can be derived from individual laser devices, such as laser diodes, or from a single laser device whose beam has been split, before modulation, into two or more beams. U.S. Pat. No. 4,884,857, issued on Dec. 5, 1989 to the same assignee as the present invention and which is herein incorporated by reference, discloses a multiple-beam laser scanning system.
One of the difficulties of multiple-beam laser scanning systems is that it is difficult to maintain exact placement of the beams in the process or in-track direction of the scan. This is needed to prevent lines or characters which have continuity in the process direction from appearing irregular. This is especially true in the case where laser diodes are used as the light producing device and the optical components of the system effectively enlarge the spacing between the diodes when imaged onto the photosensitive member.
One way to reduce the tight tolerance requirements needed for the diode spacing, and hence the laser beam spacing, is disclosed in the referenced patent. As taught therein, an aperture plate is positioned between the stationary beams and the optical system which provides the deflection necessary to scan the beams across the photosensitive member. The aperture plate contains apertures or openings through which the beams pass. The beam impinging upon an aperture is larger than the aperture, thus causing only the portion of the beam passed by the aperture to reach the photosensitive member. This effectively causes the separation between the apertures in the plate to govern the scan line spacing on the photosensitive member. The plate maintains that spacing even with some variation in the laser beam position due to physical or electrical changes in manufacturing or operation of the laser diode array.
Even though the arrangement disclosed in the referenced patent is advantageous in certain applications, it can experience the difficulty of irregular lines when the spacing provided by the aperture is not exact or when process movement of interlaced scans causes some in-track or process direction deviation from the exact position of the scanned line. The process or in-track scan direction refers to the direction in which the photosensitive member travels past the imaged laser beam. The scan or cross-track direction refers to the direction in which the beam moves across the photosensitive member as a result of the scanning action of the beam deflecting system. The terms process and scan are also defined in the referenced patent.
In order to overcome the irregularity aspect of the prior art, the present invention uses an aperture plate which has apertures designed to give other than a sharp edge to the beam which passes through the aperture. This is contrary to conventional practice for apertures. Normally an aperture is used for sharply cutting off the edges of the beam passing through the aperture. Several prior art patents show various systems which use apertures of various configurations to perform different functions, none of which is similar to the present invention. U.S. Pat. No. 4,321,630, issued on Mar. 23, 1982, and U.S. Pat. No. 4,057,342, issued on Nov. 8, 1977, are representative of references which use specially shaped slits or apertures across the width of the image to regulate or shape the image-wide light which is irradiated onto the photosensitive member or received from the original document being copied. See FIGS. 9 through 13 of U.S. Pat. No. 4,057,342 and FIG. 8 of U.S. Pat. No. 4,321,630.
U.S. Pat. No. 3,813,140, issued on May 28, 1974, and U.S. Pat. No. 4,725,729, issued on Feb. 16, 1988, both represent references which use apertures or openings having irregular shapes to compensate for other nonlinear properties of the scanning system which results in light changes being dependent upon the scan angle. See FIGS. 4 and 5 of U.S. Pat. No. 4,725,729 and FIG. 6 of U.S. Pat. No. 3,813,140. In both representative cases, the beam of light passes through the apertures at different positions, depending upon the angle of the light ray or beam which enters the aperture. U.S. Pat. Nos. 3,055,263, issued on Sep. 25, 1962, and 4,433,911, issued on Feb. 28, 1984, are representative of references which use specially shaped apertures or openings to determine distances or locations of object images passing through the aperture. See FIG. 7 of U.S. Pat. No. 3,055,263 and FIGS. 1 and 2 of U.S. Pat. No. 4,433,911.
For the reasons indicated, it is desirable, and it is an object of this invention, to provide an exposure system which improves the process direction continuity of adjacent scan lines.